1. (Field of the Invention)
The present invention relates to an improvement of the technique for heating and reducing iron oxide such as iron ore by a carbonaceous reducing agent such as coke to obtain molten metal iron, and relates to an improved process for efficiently reducing iron oxide into metal iron by simple processing, and effectively making the most of holding heat of metal iron to manufacture molten metal iron inexpensively and with excellent productivity.
2. (Description of the Related Art)
Many proposals have been heretofore made, in which a source of iron oxide such as iron ore and iron oxide is mixed with a carbonaceous reducing agent such as coke and coal to compact it into a pellet or a briquette, and the compacted substance is heated to thereby reduce iron oxide in the compacted substance to obtain metal iron.
For example, there is a process comprising: using raw materials of a pellet of fine powdery iron oxide having a solid carbonaceous material as a reducing agent internally, heating this in a rotary hearth type reducing furnace for pre-reduction, feeding the obtained pre-reduced substance from a reducing furnace to a refining furnace at a temperature of not less than 1000xc2x0 C. for refinement, and further progressing reduction to manufacture molten metal iron. This process comprises compacting a mixture of raw materials including powdery iron oxide and solid carbonaceous material into a pellet-like substance or a lump-like substance, heating the substance for about 10 to 50 minutes at a relatively low temperature of from 1100 to 1350xc2x0 C. after drying or without drying to progress reduction to about 50 to 90% of the reducing ratio, and thereafter carrying out refinement and final reduction. However, in this process, since the heating temperature at the time of reduction is low, it takes a long period of time for heating and reducing, and for realizing volume production, a large-scaled facility is necessary and in addition, the energy consumption quantity required for reduction is great.
There is a further process which comprises compacting iron oxide having a carbonaceous reducing agent into a granular or lump-like substance internally, heating and reducing the compacted substance while moving it in a horizontal direction on the moving hearth, producing and growing a metal iron shell to thereby progress reduction till iron oxide is not present therein, agglomerating produced slag within the shell of metal iron, discharging it from the moving hearth and further heating it to thereby melt the shell of metal iron, and separating it into molten slag and molten metal ion. According to this process, since heating and reducing are carried out at a high temperature of not less than 1300xc2x0 C., the time required for reduction can be considerably reduced, but since reduction should be progressed till iron oxide is not substantially presented therein after the shell of metal iron has been formed, it requires a considerable time till reduction is terminated, and accordingly, the horizontal moving distance is long, the facility becomes large-scaled and consumption energy is great. Further, in this process, since energy required for heating reduction and energy required for final reduction and melting are unbalanced, heat energy is wasteful.
There is a process for manufacturing pig iron using compact comprising iron oxide and carbonaceous reducing agent in a partially-reduced state as raw material. The compact in a partially-reduced used in this process is compact of two layer structure comprising a shell which consisted mainly of metal iron and an inner nuclear which consisted mainly of metal iron, iron oxide and free carbon. The partially-reduced compact whose total iron content in mass % is not less than 70%, metal iron content is 20 to 50%, free carbon content is not less than 5%, and volume of compact is not less than 20 cm3 is used as raw material, which is charged into a shaft furnace and heated to thereby obtain molten metal iron. The partially-reduced compact has metal iron, free carbon and iron oxide mixed therein as described above. However, the free carbon quantity is 5 to 10% which is not enough for reduction of iron oxide remained in the partially-reduced state and carburizing necessary for efficient melting so that in the final carburizing and melting step which is a next step, carbon should be added separately. Further, in this process, the partially-reduced compact is molten and reduced in a blast furnace to manufacture pig iron, but it is necessary for enhancing the processing efficiency in the blast furnace to maintain gas permiability in the furnace at a high level, for which purpose, the raw material compact should have a large diameter (for securing the volume of not less than 20 cm3 in a spherical state, a diameter is not less than about 3.4 cm), manufacturing of a large-diameter pellet or briquette as described is considerably complicated so that not only exclusive-use lump forming equipment is necessary but also the yield is poor, lacking in flexibility of raw material processing.
It is an object of the present invention to provide a process comprising: in reducing a raw material compacting substance containing a carbonaceous reducing agent and an iron oxide contained substance to manufacture metal iron, efficiently progressing reduction without involving an excessive burden in pre-compacting of raw material to obtain a partially-reduced substance; progressing, in a short period of time and efficiently, further reduction and melting in a state of maintaining the temperature of the partially-reduced substance to enable production of molten iron of high purity with excellent productivity.
The subject matter of the manufacturing process according to the present invention comprises: charging a raw material compacting substance containing a carbonaceous reducing agent and an iron oxide contained substance into a moving hearth type reducing furnace; and reducing the raw material compacting substance to the reducing ratio 30 to 80% within the reducing furnace to thereby form a shell of metal iron; or forming, with metal iron stretched around in a network fashion, a state that the carbonaceous substance remains in a clearance therein, and agglomerating FeO contained produced slag therein, taking the reduced compacted substance (partially-reduced substance) in a state of maintaining a high temperature out of the reducing furnace to charge it into an arc heating type melting furnace or an iron bath furnace; and carrying out final reduction, carburizing and melting to obtain molten metal iron.
In embodying the above process, iron oxide remained in the raw material compacting substance reduced is further reduced within the melting furnace, and the carbonaceous reducing agent carburizes reduced iron produced. However, preferably, if a carbonaceous reducing agent of surplus by 3 to 6% mass % with respect to the theoretic equivalent amount necessary for reducing iron oxide is contained in the compacted substance when the raw material compacting substance is produced, heating and reducing is progressed fully without requiring additional charge of the carbonaceous reducing agent, and later carburizing can be progressed efficiently.
If an internal temperature of a reducing furnace in which reducing is carried out is controlled in the range of from 1350 to 1450xc2x0 C., or the raw material compacting substance charged into the reducing furnace is further raised in temperature quickly to a temperature of not less than 800xc2x0 C. within 2 minutes, reducing can be progressed efficiently in a short period of time. If the temperature is controlled in a manner as described above, the aforesaid reduction of the raw material compacting substance can be progressed in an extremely short period of time, say, 3 to 5 minutes. Further, for more positively obtaining the reduction rate enhancing effect caused by the temperature control as described, it is most effective to use pellet-like or briquette-like raw material compacting substance having a diameter of 3 to 25 mm.